The clinical manifestations of 20 cases of insular cistern hematoma were characterized by sudden onset violent headache, vomiting, bloody CSF, and subhyaloid hemorrhage (1/3 cases). CT scan showed semilunar high density image centering on the insular cistern with serrate internal edge. The average volume of the hematoma was 20 ml. By means of cerebral angiography, operation and/or autopsy, 7 cases were identified as aneurysm of middle cerebral artery, 6 as aneurysm of posterior communicating artery, and 2 as amyloid angiopathy. In the other 5 cases, the etiology was unknown.